The short version: CBS originally pushed The Emmy Awards a week earlier when it saw the NFL schedule. Since it had the national double header game on 9/20, it moved the Emmy Awards to 9/13 to potentially avoid a late start. But then MTV told CBS they were locked into airing the Video Music Awards on 9/13, so CBS decided to move it back to 9/20.

via CBS press release:

EMMY(R) AWARDS MOVED BACK TO SUNDAY, SEPT. 20

The 61st PRIMETIME EMMY® AWARDS is being produced by Don Mischer Productions. Nominations for the 61st PRIMETIME EMMY® AWARDS will be announced July 16 at 5:40 AM, PT from the Television Academy's Leonard H. Goldenson Theatre in North Hollywood.

The 61st PRIMETIME EMMY® AWARDS has been re-scheduled for Sunday, Sept. 20 (8:00-11:00 PM, live ET/delayed PT) on the CBS Television Network.

"After we announced plans for September 13, MTV informed us they were locked into the same day for the Video Music Awards, with venue and sponsorship agreements in place. We had the flexibility to move; they didn't," said Jack Sussman, Executive Vice President, Specials, Music & Live Events, CBS Entertainment. "It's best for the industry and the audience that these events not compete against each other. Huge thanks to the Television Academy and Don Mischer for quickly moving mountains to make this happen. If the Emmy broadcast has as much excitement as the scheduling of the date, viewers are in for an Unforgettable event."

"We appreciate CBS and the Television Academy taking on the mammoth task of moving such a huge event as the Emmy's to accommodate the Video Music Awards. Now audiences will get to experience two phenomenal nights of entertainment," said Stephen Friedman, General Manager of MTV.

To present the 61st PRIMETIME EMMY® AWARDS as close as possible to its scheduled 8:00 PM start, 60 Minutes (7:00-8:00 PM, ET/PT) is planning a flexible broadcast following CBS Sports late afternoon NFL game.